1. Field of the Invention
The subject invention relates to the display of 4:3 aspect ratio interlaced television signals on a widescreen 16:9 television receiver.
2. Description of the Related Art
Substantially changing the deflection width of a picture tube is difficult. Relatively high power has to be handled and anode voltage is affected. The horizontal deflection of the 16:9 widescreen television receiver is therefore kept at a constant full screen width. If, however, the presented program is in the 4:3 format, then a video compression of 3/4 is needed over one line scan with the remaining 1/4 line length blanked or used for different information (for example, Picture-Outside-Picture (POP)). Alternatively, this 1/4 blanked line length could be divided into two equal 1/8 line lengths left and right of the main picture, as shown in FIG. 1.
As shown in FIG. 2, field memories are ordinarily used to convert the standard interlaced NTSC signal into a progressive scanned frame. To compress, one may choose to write into the field memory at a relatively slow rate, while still obeying Nyquist criteria, and read out at a higher rate. For pixel stability, both sample frequencies should be phase locked to each other and to the horizontal deflection.
Currently, the read out is done at a sampling rate of 910.times.f.sub.H =14.318 MHz. The progressive scan memory logic however needs two clock phases per read or write cycle and, as such, the clock rate must be doubled towards 28.636 MHz. The write frequency should then be chosen to be 3/4 .times.2.times.910.times.f.sub.H =21.477 Mhz. to obtain the desired compression. For a purely digital solution, the voltage controlled oscillator in the phase locked loop circuitry must then be brought up to three times the read frequency and divided by four to obtain the write frequency. The tripled frequency 85.9 MHz. is, however, beyond the range of present technology used in television receivers.